Brachionus rotundiformis Tschugunoff , 1921 from the Brachionus plicatilis species complex ( Rotifera : Monogononta ) : A new record from Galápagos Archipelago , Ecuador

The presence of the rotifer species Brachionus rotundiformis from the B. plicatilis species complex in Lake Arcturo, a saline lake in the Genovesa Island of the Galápagos Islands, is here reported. This is the first record of the species for the rotifer fauna of Ecuador as well as of the species complex to the Galápagos Islands. This finding is consistent with the idea of high dispersion capacity, and of cosmopolitan distribution of this species complex. Because Genovesa Island is uninhabited, passive transport by wind currents and zoochory by migrant birds seem to emerge as the most plausible factors in this process of colonization. Integrative studies on the morphological variations, genetic, molecular, and ecological aspects are still required to further understand the process of dispersion and the ecology of this member of the B. plicatilis species complex in this remote and isolated locality, and the exact taxonomical position of the island’s population to the other members of the complex. Key-Words. Brachionidae; Geographical distribution; Neotropics; Rotifers; South America.

Initially, the complex was considered to consist of two different morphologically distinguishable morphotypes, namely 'S' (small) and 'L' (large), based on the average size difference of individu-als of comparable developmental sex and age.Segers (1995) formally recognized the two as belonging to different species-level taxa, and consequently re-established the names Brachionus rotundiformis Tschugunoff, 1921 and B. plicatilis sensu stricto (s.s.) for the S and L morphotypes, respectively.
The actual number of species in the complex worldwide has been established to be at least fifteen (Mills et al., 2017;Serra & Fontaneto, 2017).The recent overview by Mills et al. (2017) has confirmed the cosmopolitan distribution of each species thus refuting the suggestion that individual species may have narrower geographic distributions, but also noted gaps in the known distribution of the complex.
Studies on rotifer fauna of continental Ecuador are scarce and relatively recent (Koste & Bötterg, 1989, 1992).After these seminal papers, only a small number of studies detailing on rotifers from inland water bodies of Ecuador has been published (De Cruz, 1998;Torres & Palacios, 2007;Quimi, 2014;Valencia, 2015).Similarly, the rotifer fauna of the Galapagos Islands has been very little studied.Only three references are available, and none mentioned the B. plicatilis species complex (De Smet, 1989;Segers, 1991;López et al., 2018).
In this paper, the presence of the rotifer B. rotundiformis from the B. plicatilis species complex is reported in Lake Arcturo, a saline crater lake located on Genovesa Island, the Galápagos Archipelago, Ecuador.This is the first record of this taxon for Ecuador and the first record of the species complex to Galápagos Islands.

Sampled locality
Analyzed material was collected at Lake Arcturo (WGS84 coordinates: 00°19'40"N, 89°57'30"W) which sits at sea level on the center of Isla Genovesa, an uninhabited, 17 km² island located in the northeastern Galápagos Archipelago (Fig. 1).The lake is circular with 500 m in diameter, 27.5 m deep at its maximum depth, and is surrounded by steep, 60 m crater walls (Fig. 2).A dense red mangrove (Rhizophora mangle Linnaeus, 1753) thicket  covers the lakeshore.The northeastern shore of the lake has a shallow-water beach area, whereas the southern part of the basin has much steeper bathymetry.Although the lake is connected to the ocean through many fissures in the island basalt a distinct seawater inlet is not obvious.Stable isotope values of water, as well as major cations and anions, indicate that the lake is hypersaline, with salinity roughly 1.5 times that of seawater (Conroy et al., 2014).The lake is frequented by large numbers of seabirds and is an outstanding example of guanotrophy.Primary production in the epilimnion is high, and the hypolimnion is usually anoxic.The lake is probably stratified during the entire year (Walker & Likens, 1975;Bryhn, 2009).There appears to be no fish in the lake.The invertebrate fauna is dominated by corixid bugs and tanaid shrimps (Howmiller, 1969).More information on this lake is given in Conroy et al. (2014Conroy et al. ( , 2015)).

Samples and examination of the material
Samples for the present study were collected on September 9 th , 2004, using a standard conical plankton net (mesh size 63 µm).They were fixed with 4% formalin solution and the individuals of the B. plicatilis complex were analyzed under a Leitz Laborlux S optical microscope.Photographs for each of the 20 individuals were taken with an adjusted camera Canon Power shot A650 IS, and morphometric measurements were obtained using ImageJ (Abramoff et al., 2004).A total of 18 lorica dimensions were measured based on Fu et al. (1991a)

RESULTS AND DISCUSSION
Based on the morphometrical analysis as well as the morphology of the examined individuals (Fig. 3, Table 1), it is evident that the taxon found corresponds to the SS-type morphological group of the species complex.More specifically our specimens are characterized by the small lorica length (131-200 µm) that falls within the range described by Ciros-Pérez et al. (2001) for B. rotundiformis, as well as the pattern of the anterodorsal spines with the second spine being shorter (g: 5-12 µm) compared to the other two (h: 7-21 µm, j: 20-29 µm) (Table 1).The anteroventral side also corresponds well with the B. rotundiformis morphology (Fig. 3).This feature has recently been proven to hold strong diagnostic features in Brachionus species (Michaloudi et al., 2017(Michaloudi et al., , 2018)).Despite the clear morphological resemblance of our specimens with B. rotundiformis we should mention that Mills et al. (2017) phylogenetically recognized two clades in the SS group, namely clade SS1 corresponding to B. rotundiformis, and clade SS2 whose morphological identity is yet unknown.
The B. plicatilis species complex is considered cosmopolitan (Serra & Fontaneto, 2017) and is found in brackish and estuarine habitats of continental Ecuador (De Cruz, 1998;Torres & Palacios, 2007) and other regions of the Neotropics (Koste & José de Paggi, 1982;José de Paggi & Koste, 1995;Vásquez et al., 1998;Garraffoni & Lourenço, 2012;Ferrando & Claps, 2016).Particularly in the Neotropical zone, the species B. rotundiformis has been reported only from Mexico (Sarma et al., 2000), Brazil (Garraffoni & Lourenço, 2012) and Argentina (Ferrando & Claps, 2016).Of the two clades proposed by Mills et al. (2017) phylogenetically recognized in the SS group, the  The presence of B. rotundiformis in Lake Arcturo in Genovesa Island is consistent with the great dispersal ability in this species complex (Serra & Fontaneto, 2017).Among the mechanisms of dispersion of rotifers indicated by Segers & De Smet (2008), passive transport by wind currents and zoochory by migrant birds emerge as most plausible factors in the process of colonization.In the Galápagos Archipelago, about 27 species of migrant birds are found (Windenfeld, 2006) and particularly Lake Arcturo is inhabited by large seabird populations (Howmiller, 1969).Since Genovesa Island is uninhabited, the effects of human activities are reduced to visiting tourists.However, the number of human visitors to the islands has increased exponentially in the last 30 years and may be associated with the recent colonization of Keratella cochlearis in San Cristóbal Island (López et al., 2018).Still in the case of Genovesa, we hypothesize that human activities are less important in the introduction of rotifers compared to the other inhabited islands.More studies are required to understand the process of dispersion of this complex species in Galápagos islands.Due to the remoteness and isolation of the studied locality, this record of B. rotundiformis is particularly important in relation to the processes of dispersion and speciation of the species complex.

Figure 1 .
Figure 1.Map of the Galápagos Archipelago showing the location of the Genovesa Island and Lake Arcturo.
,Ciros-Pérez et al. (2001),Proios et al. (2014), andMichaloudi et al. (2017).Examined material is deposited in the rotifers collection of Dr. Evangelia Michaloudi in Department of Zoology, Aristotle University of Thessaloniki, Thessaloniki, Greece.Other voucher specimens were deposited in the collection of aquatic invertebrates of Dr. Miriam Steinitz-Kannan in Department of Biological Sciences, Northern Kentucky University, Highland Heights, Kentucky, U.S.A, and in the rotifer collection of Dr. Carlos López in Escuela de Acuicultura y Pesquería, Universidad Técnica de Manabí, Bahía de Caráquez, Ecuador.